1. Field of the Invention
The present invention relates generally to stabilizers for polymers based on halogen-containing vinyl monomers and, more particularly, to stabilizer mixtures which, in addition to calcium and zinc soaps, also contain optionally surface-modified garnets.
2. Discussion of Related Art
It is known that halogen-containing plastics or even the molding compounds produced from them tend to eliminate hydrogen chloride under the effect of heat and UV light, which promotes the decomposition and discoloration of the halogen-containing plastic or molding compound. Hitherto, the traditional heavy metal compounds based on lead, barium and/or cadmium have been used to counteract this decomposition and discoloration. However, from the industrial hygiene and ecological point of view, there is a need to replace these entirely effective stabilizers by safer compounds. Although stabilizer mixtures containing calcium and zinc soaps have established themselves in this regard over recent years, they always require additional co-stabilizers to develop an acceptable performance level. The co-stabilizers may be both inorganic and organic. Hydrotalcites with small BET surfaces are proposed as inorganic co-stabilizers in DE-C-30 19 632. Hydrotalcites are layer compounds which generally contain layers of magnesium-containing hydroxyaluminates in which magnesium and aluminium ions are statistically distributed, the hydroxyl groups often being partly replaced by carbonate groups. In order to improve the compatibility of these hydrotalcites in the halogen-containing plastics, it is recommended in the published German patent application to surface-modify the hydrotalcites with fatty acid esters, anionic surfactants or coupling agents. Although hydrotalcites are effective stabilizers, they are difficult to produce and, hence, are expensive.
It is known from DE-C-39 41 902 that basic calcium/aluminium hydroxyphosphites can be used to stabilize PVC. The calcium aluminates described in this document also appear to be layer compounds like the hydrotalcites. Besides these two types of layer compounds, calcium silicates, titanates and aluminates are known as stabilizer constituents for PVC from DE-C-27 16 389 and from DE-B-25 53 409. In many cases, however, these compounds cannot be adequately dispersed in PVC and, in addition, show a poor stabilizing effect.
The problem addressed by the present invention was to provide a stabilizer for polymers based on halogencontaining vinyl monomers which would be easy to produce and which could readily be dispersed in the plastic. In addition, the stabilizers would enhance the stabilizing effect of calcium and zinc soaps so that the plastic would retain a good light initial color for long periods under the effect of heat and/or UV light, only becoming dark in color at a much later stage, indicating the failure of the stabilizer mixture.